Before the failure of the German revolution, the leaders of the Russian revolution had counted on its victory, and the subsequent victory of a sweeping international revolution. The hope was that, since Germany was among the foremost industrial powers of the world, it could provide material necessities and alleviate the stresses of war, in turn allowing for demilitarization and democratization across both Germany and Russia. Since this didn't happen, the ascendant Bolsheviks were left in a very, very difficult situation. They were in charge of a war-weary nation with hostile states on all sides and powerful counterrevolutionaries within. Realistically there was only one thing they could do besides capitulate-- dig in, and use the full power of the state to survive for as long as possible.
Rosa Luxemburg summarizes this impasse pretty well in The Russian Tragedy:
The awkward position that the Bolsheviks are in today, however, is, together with most of their mistakes, a consequence of basic insolubility of the problem posed to them by the international, above all the German, proletariat. To carry out the dictatorship of the proletariat and a socialist revolution in a single country surrounded by reactionary imperialist rule and in the fury of the bloodiest world war in human history – that is squaring the circle. Any socialist party would have to fail in this task and perish – whether or not it made self-renunciation the guiding star of its policies. ... Such is the false logic of the objective situation: any socialist party that came to power in Russia today must pursue the wrong tactics so long as it, as part of the international proletarian army, is left in the lurch by the main body of this army.